1. Field of the Invention
The present invention relates to a spread spectrum communication system which is applicable to various radio (wireless) network systems, and particularly to a spread spectrum communication system which can be surely operated even under an unfavorable (severe) radio-wave condition.
2. Description of the Related Art
As a system for performing data communications in a radio (wireless) fashion are well known various systems in which a personal computer and a printer, for example, are linked to each other through a radio circuit, and print data created by the personal computer are transmitted to the printer to carry out a print operation. In order to exclude an effect of fading and ensure a desired communication distance, the above systems have frequently used a spread spectrum communication system (SSC) in which a spread spectrum (SS) modulation system is used as a system for modulating electric waves associated with information to be transmitted.
In the above-described spread spectrum communication system, at a transmission side the spectrum (frequency components) of an information signal to be transmitted is spread (bandwidth is increased) by using a PN (Pseudo Noise) code which has a much higher variation rate (i.e., a higher chip rate) than the information signal (i.e., bit rate). Further, at a reception side, a reference PN code which is coincident with the PN code of the transmission side is used, and the PN code of the transmission side and the reference PN code of the reception side are correlated with each other to demodulate the original information signal.
The PN code is a kind of cipher which is composed of a combination (sequence) of digital signals xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d. For example, a PN code having a long period has an irregular repetitive pattern of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d, and thus it appears to be noise. Therefore, a third party cannot intercept an information signal mixed with such a PN code.
However, the above-described data communication system has a problem that a destination to which data is to be transmitted cannot be identified unless the data are demodulated. In the data communication system as described above, the data are generally designed in an 8-bit data configuration, and address information representing a destination to which the data are to be transmitted is contained in the data portion. Accordingly, the reception side cannot check whether the data are addressed thereto unless the reception side checks the address information in the data portion decoded (demodulated) after synchronization acquisition. This is a factor of lowering the data access efficiency. In addition, since the data are designed in an 8-bit data configuration as described above, the number of destinations to which data can be transmitted, that is, the number of channels is limited to 256 (=28).
In order to solve the above problem, Japanese Patent Application No. Hei-6-308293 (Japanese Laid-open Patent Application No. Hei-8-149047) proposes a technique for detecting a destination at a high speed without demodulating data.
According to this technique, a code representing a destination is input in a preamble locating before data to be transmitted, and code synchronization is carried out before demodulation of the data in order to identify the destination. Here, the code synchronization is defined as follows. When a transmission side transmits to a reception side a code group having PN codes which are different every period, the reception side receives the code group in time sequence to collate the code group with the reference PN code corresponding to the code group (i.e., upon viewing from the reception side, special PN code sequences each of which is allocated to each reception side are transmitted). In this case, if correlation peaks between the code group and the reference PN code can be obtained over all the periods (i.e., four periods), the reception side can automatically recognize that the signal containing the code group is addressed to the reception side itself (i.e., the code synchronization is established). Here, the code group is predetermined for each reception side (receiver), and each code group corresponds to a channel. For example, assuming that the code group comprises PN codes of four periods and each PN code has 31 chips at one period, totally 312=923521 logical channels can be formed.
In this case, some threshold level must be set to detect correlation peaks between the signal transmitted from the transmission side and the signal at the reception side. However, under adversely varying electric-wave environments, the setting of the threshold value is difficult, and thus a malfunction occurs with high probability. Further, when the detection of the correlation peak between the signal at the transmission side and the signal at the reception side is performed with a threshold level, the setting of the level is cumbersome, and there may occur a case where a signal does not exceed the threshold level although it is a right signal.
Further, in this technique, in addition to the destination identifying code described above, a code for synchronization acquisition is also contained in the preamble, that is, plural codes are provided to the preamble, so that spurious occurs at the code switching time.
An object of the present invention is to provide a spread spectrum communication system which can solve the above problem of the prior art by using a transmission signal which is constructed so as to enable high-speed media access, so that a destination to which data is to be transmitted can be specified prior to demodulation of the data even under an unfavorable electric-wave environment.
In order to attain the above object, according to a first aspect of the present invention, a spread spectrum communication system includes a transmission side for performing spread-spectrum modulation using a pseudo noise (PN) code on a frame-structured transmission signal having at least one preamble and a data portion subsequent to the at least one preamble, and a reception side for receiving the transmission signal which is subjected to the spread-spectrum modulation at the transmission side, wherein the reception side includes destination identifying means for detecting the periodicity of the correlation peaks between the PN code in the at least one preamble of the transmission signal and a reference pseudo noise code generated at said reception side to count a prescribed number of correlation peaks, and checking on the basis of the counted number of correlation peaks whether the transmission signal is addressed to the reception side.
According to a second aspect of the present invention, a spread spectrum communication system includes a transmitter for performing spread-spectrum modulation using a pseudo noise (PN) code on a frame-structured transmission signal having data to be transmitted subsequently to at least one preamble(s), and reception means for receiving the transmission signal which has been subjected to the spread-spectrum modulation, wherein the reception means includes means for detecting the periodicity of the correlation peaks between the PN code in the preamble(s) of the transmission signal and a reference pseudo noise code to count a prescribed number of correlation peaks.
According to the first and second aspects of the present invention, the above-described spread spectrum communication system is provided with the means for detecting the periodicity of the correlation peaks between the PN code in the preamble(s) of the transmission signal and the reference PN code without setting a threshold level and counting the prescribed number of correlation peaks. Upon counting the prescribed number of correlation peaks (i.e., detecting the continuous periodicity of the correlation peaks between the code group of the preamble(s) and the reference PN code), the reception side can automatically specify the transmission destination (i.e., the reception side can recognize that the transmission signal containing the preamble(s) is address to the reception side itself). Accordingly, the communications can be performed by merely counting the number of correlation peaks before the data portion is demodulated. Therefore, even under an unfavorable electric-wave environment, high-speed media access can be surely performed.
In the above-described spread spectrum communication system, plural preambles may be provided to the transmission signal. In this case, the respective PN codes in the respective preambles may be designed to be coincident with one another, but to be shifted in phase from one another.
In the above case, the transmission signal is provided with the plural preambles, and the periodicity of the correlation peaks between the PN code in each preamble and the reference PN code is detected to count the prescribed number of correlation peaks. Accordingly, even when an interfering electric wave has some periodicity, a transmission destination can be specified for data communication before the data portion is demodulated, and high-speed media access can be accurately performed even under an unfavorable electric-wave environment. The reason is as follows. The PN codes of any two preambles (adjacent two preambles) are set to be different in phase, and thus the appearance time (position) of each correlation peak is different between the preambles because of the difference in phase of the preambles. Accordingly, the behavior of the appearance time (position) of the correlation peaks which is caused by the phase difference between the preambles is different from that of the correlation peaks caused by the periodic interfering electric wave.